Method for carrying out a keyless access authorization check and keyless access authorization check device

ABSTRACT

A method and device for checking keyless vehicle access authorization of an operator. The method includes transmitting a code signal from a base station to a mobile identification (ID) transmitter carried by the operator. In response to receiving the code signal, the ID transmitter performs an action which is indicative of a reply signal that is detectable by the base station. The difference of a signal characteristic between a reference code signal monitored at the base station and the reply signal received by the base station is then determined. The signal characteristic correlates with signal propagation time and changes as a function of the distance between the base station and the ID transmitter. The distance between the base station and the ID transmitter is then determined on the basis of a relative determination based on the difference of the signal characteristic between the reference code signal and the reply signal.

TECHNICAL FIELD

The invention pertains to the field of carrying out keyless accessauthorization control for checking, for example, whether a person isauthorized to perform a certain action, such as opening a vehicle door.In particular, the invention pertains to a method for carrying outkeyless access authorization control by means of wireless communicationbetween a transceiver unit corresponding to a base station and a mobileidentification transmitter (ID transmitter) in order to check the accessauthorization of the person carrying the ID transmitter. The methodincludes: transmitting a code signal from the transceiver unit of thebase station, receiving this code signal by the ID transmitter, carryingout an action that can be detected by the transceiver unit and that isexecuted by the ID transmitter at the receipt of the code signal as areply signal of the ID transmitter, and detecting the action of the IDtransmitter and determining the distance between the base station andthe ID transmitter.

The invention also pertains to a keyless access authorization controldevice with a base station featuring a transceiver unit as well as acontrol and evaluation element and a mobile identification transmitter(ID transmitter) featuring a transceiver unit for checking the accessauthorization of the person carrying the ID transmitter. This basestation can calculate the distance from the base station to the IDtransmitter.

BACKGROUND ART

Keyless locking systems are used in numerous applications, for example,in vehicles to increase user convenience. Conventionally, infrared andradio systems are used as remote-control systems wherein the authorizeduser activates the ID transmitter in order to transmit a signal to thebase station, for example, to a receiver unit in a vehicle for openingthe vehicle. To further increase user convenience, passive accessauthorization control systems can be used as the user passes betweenvehicles, so that an authorized user carrying a valid ID transmitter canopen his vehicle without having to actively turn on the ID transmitter.Such a keyless access authorization control device is described, forexample, in DE 43 29 697 C2. In the method disclosed in this document, acode signal is transmitted from a transceiver unit in a vehicle, andthis signal is received by the ID transmitter when the transmitter iswithin the reception area of this signal. Then, a reply signal is sentback from the ID transmitter due to the action triggered by the receiptof the code signal. The validity of the reply signal is checked after itis received by the receiver unit of the vehicle. The receipt of a validreply signal triggers the desired action in the vehicle, namelyunlocking the vehicle doors. This passive system is problematic in thatthe transmission path between the base station and the ID transmittercan be extended by simple means that are unauthorized and unnoticeableto the authorized user. In this way, an unauthorized person canestablish radio connection between the transceiver unit of the vehicleand the ID transmitter authorized for opening, even if the latter is notwithin the reception area of the request signal. Thus, the unauthorizedperson can gain unknown entry to the vehicle.

The object of DE 196 32 025 A1 addresses this problem. In the keylessaccess authorization control device described in this document,detection of the propagation time relative to the transmitted requestsignal and the received reply signal is carried out, in which thecorresponding time span required by the transmitted code signal to betransmitted from a base station and then received as a reply signal iscalculated. If the transmission path has been lengthened, then the timeinterval from the transmission of the code signal to the receipt of thereply signal is naturally greater than when the ID transmitter isdefinitely in the direct vehicle proximity and the corresponding returnradio path is short. If the calculated signal propagation time exceeds apredefined value, the access authorization control method is stopped inorder to prevent unauthorized entry.

In order to carry out the access authorization control method describedin this document, it is necessary to detect the signal propagation timewith high accuracy. Communication between the transceiver unit and themobile ID transmitter is typically within a very limited area around thebase station: for vehicles, an area between 5 and 10 m e.g. Thepropagation time of an ordinary transmitted and received signal withinthis distance is between 16.5 ns and 33 ns. The detection of such shorttime spans, in particular with the desired resolution, is only possiblewith significant expense, an expense that is not justifiable fornumerous applications, for example, for a vehicle.

SUMMARY OF THE INVENTION

Starting with the previously discussed prior art, the task of theinvention is based on proposing such a method for carrying out keylessaccess authorization control that not only features sufficient securityrelative to the possibility of detecting a manipulated path extension,but that can also be realized with the use of simple means.

The task of the invention is also based on proposing such a keylessaccess authorization control device that can detect a (manipulated) pathextension with the use of justifiable means.

The task related to the method is solved according to the invention inthat the distance determination is done based on a relativedetermination, in which the difference of a distance-dependent signalcharacteristic correlated with the signal propagation time between areference code signal monitored in the base station and the reply signalreceived by the transceiver is evaluated.

The knowledge of the claimed invention is based on the fact that it ispossible to determine distance with sufficient resolution over indirectpaths by detecting a signal value correlated with the propagation timeof a signal using simple means. In contrast to the previously knownmethod, this distance determination is done by means of a relativedetermination, in which a reference code signal is monitored in the basestation with reference to the transmitted signal characteristic and thesignal characteristic of this reference code signal is compared with thesignal characteristic of the reply signal returned by the ID transmitterand received by the transceiver unit. The distance-dependent signalcharacteristic to be compared with the reference code signal givesinformation about the return path of the code signal and reply signalwhen compared with the corresponding reference code signal. In this way,the reference code signal, for example, the transmitted code signal, canbe monitored during the transmission period of the code signal by thebase station. The detection by the base station of the actions executedby the ID transmitter is limited by the measurement time distance. Theseactions can be, for example, the return transmission of the code signalreceived by the ID transmitter. The actions executed by the IDtransmitter can also be other actions that can be detected by the basestation, such as the shutdown of a transmitter and thus the detection bythe base station of the time when such a transmitter is turned off bythe ID transmitter. The use of hardware to carry out such a relativedistance determination is very minimal in comparison with that requiredto carry out an absolute propagation time measurement. Depending on themethod used, numerous evaluation steps can also be eliminated duringdata processing.

Starting with the background of this embodiment and the description ofthe invention, either the delay times by the circuit elements used aremuch shorter relative to the actual signal propagation time or they areknown, or they are calculated and taken into consideration in theevaluation.

To carry out such a relative distance determination, different signalcharacteristics can be used. For example, counting the cycles of thecarrier wave of the transmitted code signal over a time period until,for example, a reply signal has been received by the transceiver unitcan be done in the base station. The number of cycles counted in thebase station over the time until the detection of the performed action,for example, the return transmission of the received code signal,multiplied by the period of the carrier wave gives the propagation timeof the signal from the base station to the ID transmitter and back. Thedistance between the base station and the ID transmitter is then givenby half the signal propagation time multiplied by the speed of light.Thus, the propagation time cannot be detected in continuous increments,but rather in multiplies of the period. Using a carrier frequency of 100MHz, for example, the resulting distance determination between the basestation and the ID transmitter has a resolving power of approximately1.5 m. Using a carrier wave in an ISM band, with an approximatefrequency of 434 MHz, a resolving power of approximately 0.35 m can beachieved due to the correspondingly smaller period.

To realize the method according to the invention using thisconfiguration, electronic components that are otherwise used to performauthorization requests can be used. Additional requirements are merelythe use of a counter that can be designed, for example, out of logicgates or flip-flops. Thus, the method according to the invention can berealized without great additional expense.

Expediently, the wireless communication is performed in a frequencydistance within an ISM band. The resolving power of the distancedetermination during use of such a frequency is considerably higher thanthat required for the use of such a method for keyless accessauthorization control for a vehicle. The resolving power for using sucha method for vehicles is on the order of 5-10 m. Due to the minimalresolving power required in such an application, compared with thepossible resolving powers, the requirements on the counter can belessened such that not every cycle, but rather every xth, orapproximately every 2^(x)th cycle, is counted. Such selective cyclecounting can be realized through the use of a frequency divider or afrequency mixer, so that for a dividing ratio of 1:16, only every 16thcycle is counted. Using a frequency of 434 MHz and a dividing ratio of1:16 results in a counting frequency of 27.125 MHz and consequently aresolving power of approximately 5.5 m; a resolving power that issufficient for using the method for vehicles, relative to distancedetermination.

In a refinement of this embodiment, the command TRANSMITTER IDTRANSMITTER ON is transmitted by the transceiver in a first step fordistance determination. Based on this command, a transmitter is turnedon in the ID transmitter in order to transmit at a first frequency.After a predetermined length of time that is sufficient for thetransmitter of the ID transmitter to have transmitted, a code signal istransmitted at a second frequency by the transceiver unit of the basestation. This code signal is the command TRANSMITTER ID TRANSMITTER OFF.Simultaneous with the transmission of the command, the cycle count ofthe carrier wave of this transmission signal begins. Beforehand, thereceiver channel of the transceiver unit is switched to the firstfrequency so that the signal transmitted by the ID transmitter can bereceived. The cycle count is stopped when the transmission signal of theID transmitter can no longer be detected by the transceiver unit of thebase station. Thus, the cycle count is performed for a length of timenecessary for the propagation time of the code signal from the basestation to the ID transmitter and back. The action signal of the IDtransmitter detected by the base station is the reaction of the IDtransmitter to the transmitted code signal.

In another configuration of this embodiment, the code signal transmittedby the base station is returned by the ID transmitter in a mirror-likefashion. In this embodiment, the cycle count is stopped when the codesignal as the reply signal of the ID transmitter has been completelyreceived again by the transceiver unit of the base station. In anotherembodiment, in order to be able to compensate for possible data lossduring data transmission, two cycle counters are assigned in the basestation, wherein a first cycle count is coupled to the transmission ofthe code signal and another cycle count is coupled to the receipt of thecode signal reflected by the ID transmitter. The cycle count of thesecond counter coupled to the receipt of the code signal is stopped whenthe counter associated with the transmission of the code signal reachesa count that corresponds to twice the number of cycles corresponding tothe code signal. The greater the distance between the base station andthe ID transmitter, and correspondingly, the longer the signalpropagation time, the larger is the difference between the calculatedcounter status associated with the receipt of the code signal and thenumber of cycles corresponding to the code signal. For the last twoexamples, it is expedient to transmit a predetermined number of cyclesof a carrier wave.

Another embodiment of the invention takes advantage of the fact that thephase of a wave transmitted over a radio path is shifted depending onthe return radio path relative to the originally transmitted phase. Inthis way, both the phase of the frequency and also the amplitudevariation (envelope) can be used individually or together in order tocompare the phase of the transmitted request signal with thecorresponding reply signal returned by the ID transmitter. In this way,the reply signal is transmitted from the ID transmitter to thetransceiver of the base station using a carrier wave. The carrier wavecan be a wave within the reply signal, for example, the request signalitself, or it can be transmitted to the ID transmitter by modulating acarrier wave for transmitting the request signal. If the ID transmitteris located within the predetermined receive area of the transmittedrequest signal, which is less than 3 m for the application of the methodto the automotive field, the return path (transceiver unit-IDtransmitter-transceiver unit) is short. The phase shift between thedemodulated reply signal and the originally transmitted request signalis so small that it can be produced within a predetermined tolerancedistance of phase-locked signals (request signal and reply signal). Fora manipulated path extension, the phase between request signal and replysignal is constantly shifted, so that this is outside the tolerancedistance of the operation defined as phase-locked. The probability ofthe received and demodulated reply signal having a phase correspondingto the respective request signal is purely random and thus it isextremely small. Consequently, the desired action, like the opening of avehicle, is only performed when the ID transmitter is at thepredetermined distance to the base station.

To prevent undesired feedback, the request signal is expedientlytransmitted at a different frequency than the reply signal. In anespecially simple configuration, the request signal is transmitted atthe frequency for modulating the reply signal. This can be realized sothat, for example, the request signal is transmitted on a low-frequencychannel and this request signal received by the ID transmitter is usedto directly modulate a carrier wave on a high-frequency channel. Forexample, a request signal can be a wake signal transmitted on an LFchannel for switching the ID transmitter from quiet or sleep mode toactive mode.

To further increase access security, the oscillator for transmitting therequest signal is operated in a free-running manner, so that it producesknown deviations in frequency. In addition, the transmission frequencycan be changed according to a predetermined variation pattern so thatthe chance of realizing an unauthorized and yet phase-locked pathextension is even further reduced.

In another configuration of this embodiment, a modulated carrier wavewith a modulation scheme of one or more modulation variables istransmitted as a request signal by the transceiver unit. In this way,either a modulation scheme of different frequencies or differentamplitudes or even a scheme of these two values is used. After receiptof the reply signal modulated in this way, filtering relative to theoriginal modulation scheme components, for example, the individualfrequency components, is performed after demodulation of the replysignal. The subsequent step of the phase comparison of the originallytransmitted request signal with the received reply signal is thenperformed with reference to the individual components forming themodulation scheme, for example, the frequency components. The use, forexample, of a modulation scheme of different frequencies for modulatingthe request signal increases the operation security of the claimedmethod, in particular, to the effect that a chance phase equalizationfor an unauthorized path extension can be recognized due to thepossibility of an absolute distance measurement of the return radiopath. For the use of a frequency scheme, a base frequency is used andmixed expediently with a defined number of other frequencies that areeach components of the base frequency divided by two. This can berealized in a simple way with a frequency divider. In this way, thevalue of the base frequency defines the resolution of the distancemeasurement and the number of divisions of the maximum detectabledistance.

A phase comparison between the originally transmitted request signal andthe received reply signal can be realized, for example, through the useof a phase comparator.

In another embodiment of the invention, a modulated carrier wavetransmitted over a radio path can be evaluated depending on the lengthof the return radio path with reference to a change or replacement ofthe function value of the used modulation variables compared withsimultaneous function values of the originally transmitted signal. Thismethod takes advantage of the time-dependent shift of the modulationcurve of the reply signal relative to the identical modulation curve ofthe request signal due to the distance between the base station and IDtransmitter. According to the claimed method, a function value of themodulation variables of the received reply signal at a certain time iscompared with the simultaneous function value of the modulationvariables of the originally transmitted request signal. Such acomparison is expediently performed by the step of a differenceformation of absolute values or also squares of the function values ofthe modulation variables at a predetermined time. This function valuecomparison can be performed at quasi-arbitrary positions of themodulation function and thus, continuously.

To increase access security, the frequency of the carrier wave for therequest signal is different from that of the corresponding reply signal.It is expedient to transmit the request and/or reply signal onrespective paths, on which there occurs an encrypted data dialog betweenthe base station and the ID transmitter, if necessary.

In one configuration of this method, the carrier wave is transmittedfrequency modulated, for easier evaluation, preferably linearlyfrequency-modulated. Correspondingly, the base station has a means fortransmitting such a carrier wave as a request signal, as well as an FMdemodulator that performs a demodulation of the received reply signal.The mobile ID transmitter has an FM demodulator for demodulating thereceived request signal. The output of the demodulator is expedientlyconnected to an input of the transceiver for data communication, so thatthe data signal transmitted from the ID transmitter simultaneouslyrepresents or contains the reply signal to the transmitted requestsignal due to its modulation.

The comparison of a function value of the modulation variables, forexample, the modulation frequency of the received reply signal, with thesimultaneous function value of the modulation frequency results in adifference frequency. The measurement of this difference frequency is ameasure of the length of the return radio path (base station-IDtransmitter-base station). Authorization control is expedientlyperformed such that a threshold for the difference frequency is preset,wherein when this threshold is exceeded, the authorization is denied.

Alternative to the use of a frequency-modulated carrier wave, anamplitude-modulated carrier wave or also a frequency and amplitudemodulated carrier wave can be used to characterize the request and replysignal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, a schematic block circuit diagram of a keyless accessauthorization control device with a base station and an ID transmitter,

FIG. 2, a time sequence diagram for determining the distance between thebase station and the ID transmitter of FIG. 1,

FIG. 3, a schematic block circuit diagram of an additional keylessaccess authorization control device containing a base station and an IDtransmitter,

FIG. 4, a schematic block circuit diagram of another keyless accessauthorization control device containing a base station and an IDtransmitter,

FIG. 5, a time sequence diagram for determining the distance between thebase station and the ID transmitter corresponding to the embodiments ofFIGS. 3 or 4,

FIG. 6, a schematic block circuit diagram of a keyless accessauthorization control device,

FIG. 7, a principle sequence schematic for checking the accessauthorization of a person carrying an ID transmitter,

FIG. 8, a schematic block circuit diagram of another keyless accessauthorization control device,

FIG. 9a, a diagram with a base frequency and two additional frequencycomponents,

FIG. 9b, a diagram of the amplitude curve resulting from the frequencymixing of FIG. 9a,

FIG. 10, a block circuit diagram of a transceiver of a base station,like that for a vehicle,

FIG. 11, a schematic block circuit diagram of an ID transmitter, and

FIG. 12, a diagram with a linearly frequency-modulated carrier wavedistributed over time as the transmitted request signal and the receivedreply signal shifted in time relative to the request signal.

BEST MODES FOR CARRYING OUT THE INVENTION

A keyless access authorization control device 1 includes a base station2 and a mobile identification transmitter (ID transmitter) 3. Atransceiver unit 4 with an HF transmitter 5 and an HF receiver 6 thatoperate at different frequencies belong to the base station 2. The basestation is also connected to a microcontroller 7 for controlling thetransmission operation and for evaluating the received signals. A cyclecounter 8 for counting the cycles of the received carrier wave isconnected to the HF receiver 6. The cycle counter 8 has a start input 9and a stop input 10. The start input 9 is connected to themicrocontroller 7. The start signal for starting the cycle counting istransmitted to the cycle counter 8 through the start input. The stopinput 10 is connected to an AND-gate 11, the HF receiver 6, and also tothe microcontroller 7. After the receipt of a signal by the HF receiver6, a stop signal for stopping the cycle counting is applied to stopinput 10 of the cycle counter 8 when no more signals are received.

The ID transmitter 3 also includes a transceiver unit 12 with an HFtransmitter 13 and an HF receiver 14. The HF transmitter 13 operates atthe same frequency as the HF receiver 6 of base station 2; the HFreceiver 14 of ID transmitter 3 operates at the same frequency as the HFtransmitter 5 of base station 2. Thus, the HF path between the basestation 2 and the ID transmitter 3 is duplex capable. The HF receiver 14is connected to a microcontroller 15 for evaluating incoming signals.The microcontroller 15 is connected to the HF transmitter 13 by means ofa data line 16 and a key line 17. Parallel to the microcontroller 15,evaluation logic 18 is arranged in the ID transmitter 3 between the HFreceiver 14 and the HF transmitter 13. The evaluation logic 18 is usedto recognize a code signal transmitted from the base station 2, as wellas to control HF transmitter 13 directly; without requiring such asignal to be evaluated by the microcontroller 15 and the HF transmitter13 to be controlled. Because only a completely defined code signal mustbe recognized by the evaluation logic 18, the required time forevaluation is constant in contrast to the required processing time ofthe microcontroller 15. The evaluation logic 18 is connected to themicrocontroller 15 for turning the evaluation logic 18 on and off bymeans of a control line.

The keyless access authorization control device 1 shown in FIG. 1 isused to carry out an encrypted request-reply dialog for determiningaccess authorization and also to determine the current distance of IDtransmitter 3 from base station 2. The distance determination betweenbase station 2 and ID transmitter 3 is performed as follows, wherereference is made to the sequence diagram of FIG. 2: within acommunications telegram in the frame of the stated encryptedauthorization control, the command TRANSMITTER ID TRANSMITTER ON istransmitted to ID transmitter 3 by HF transmitter 5 of base station 2 ata predetermined time t₀. At time t_(a), the HF receiver 14 of IDtransmitter 3 receives this command. At time t_(b), the HF transmitter13 starts to transmit a carrier wave at the frequency f₁. The carrierwave transmitted at the frequency f₁, is received by HF receiver 6 ofthe base station at time t_(c). Upon receipt of the carrier wave, thecycle counter 8 is set to zero. At time t₁, a code signal, namely thecommand TRANSMITTER ID TRANSMITTER OFF, is transmitted by HF transmitter5, and simultaneously, the cycle counter 8 for counting the cyclesreceived at frequency f₁, is started by means of start input 9. The codesignal has been transmitted by HF transmitter 5 of base station 2 atfrequency f₂. After receipt of the code signal by HF receiver 14 of IDtransmitter 3 and after recognition of this command by evaluation logic18, the HF transmitter 13 of ID transmitter 3 is turned off at the timelabeled t_(d) in FIG. 2. Starting at time t_(c), HF receiver 6 of basestation 2 receives the carrier wave transmitted by HF transmitter 13 ofID transmitter 3. After HF transmitter 13 is turned off, HF receiver 6can no longer receive the carrier wave at time t_(e). At this time, thecycle counter 8 for determining the count state is also stopped. Now,the detected cycle count state is a measurement of the propagation timeof the signal between base station 2 and ID transmitter 3 and back. Forcalculating the propagation time and the resulting distance, the idletime and also the signal lengths are taken into consideration.

FIG. 3 shows a block circuit diagram of another keyless accessauthorization control device 19 with a base station 20 and an IDtransmitter 21. In this figure, only the components are shown that arenecessary to determine the distance between base station 20 and IDtransmitter 21. The shown module is partially integrated withconventional hardware in order to realize an encrypted accessauthorization control. Base station 20 includes an antenna 22 as atransceiver unit for transmitting and receiving signals. These signalsare based on a carrier wave. The carrier wave is generated in a signalgenerator 23 and is then applied to a signal splitter 24. The signalsplitter 24 supplies the generated signal, on the one hand, to theantenna 22, and on the other hand, to a first counter circuit 25. HFswitch 26 is used to switch the operation of base station 20 betweentransmission, as shown in FIG. 3, and reception. At the receipt of thecode signal returned by ID transmitter 21, the HF switch 26 is set toits other position, in which a second counter circuit 27 is connected toHF switch 26. Both counter circuits 25 and 27 have a counter 28 and 29,respectively. Counters 28, 29 are connected to a microcontroller 30.

The signal generated by the signal generator 23 is a carrier wave at afrequency within an ISM band, namely 434 MHz. To reduce the requirementson the counter 28, a frequency dividing circuit 31, which consists of anactual frequency divider 32 with a dividing ratio of 1:16, a bandpassfilter 33 connected to the output of the frequency divider, and anamplifier 34 are connected to the input of the counter. Acorrespondingly designed frequency circuit 35 is associated with thecounter circuit 27.

The ID transmitter 21 also has an antenna 36 as a transceiver unit thatis connected at its output to HF switch 37. In receive mode, as shown inFIG. 3, an amplifier 38 and another HF switch 39 is connected to theoutput of HF switch 37. The received code signal, the carrier wave, isapplied to counter 40. A frequency dividing circuit 41 corresponding tofrequency dividing circuits 25 and 27 of base station 20 is connected tothe input of this counter. Parallel to the amplifier 38, a signalgenerator 43 is connected by means of a signal splitter 42, whosesignals can be transmitted by antenna 36 according to the correspondingpositions of HF switches 37, 39

FIG. 4 shows another keyless access authorization control device 44 thatis fundamentally designed like the keyless access authorization controldevice 19 shown in FIG. 3. Identical elements of the two accessauthorization control devices 19 and 44, respectively, are labeled withthe same reference numerals. In contrast to access authorization controldevice 19, the keyless access authorization control device 44 has afrequency mixer consisting of a local oscillator 45 and a mixer 46instead of the actual frequency divider 32.

In the following, the function of the keyless access authorizationcontrol device 19 is described with reference to determining thedistance from the base station 20 to the ID transmitter 21 ,wherereference will also be made to FIG. 5. Corresponding parts also apply tokeyless access authorization control device 44 of FIG. 4. A constant HFcarrier is generated by signal generator 23. At time t₀, the counter 28is set to 0 and switched to count the cycles of the HF carrier, where HFswitch 26 is simultaneously switched to transmission. At time t₁, whichis shifted from time t₀ due to the signal propagation time between thebase station 20 and the ID transmitter 21, the HF carrier wave isreceived by ID transmitter 21. A predetermined number of cycles, namely2048 cycles, is used as the code signal. At time t₂, that is, aftertransmission of the code signal, the base station is switched to receiveby means of HF switch 26, so that further transmission of the HF carrierwave is cut off. At time t₃, the count of counter 40 of ID transmitter21 corresponds to the transmitted number of cycles, namely 2048. At thistime, the ID transmitter switches from receive mode to transmit mode, sothat the two HF switches 37 and 39 are each in their other positions,which are not shown in FIG. 3. The signal generator 43 of ID transmitter21 is turned on beforehand so that a constant HF carrier is availablefor transmission. The frequency of the HF carrier generated by signalgenerator 43 corresponds to signal generator 23 of base station 20. Atthis point, the code signal, namely, 2048 cycles of the carrier, istransmitted back by ID transmitter 21 beginning at time t₃. According tothe signal propagation time, the beginning of the return transmission ofthe code signal is received shifted in time by base station 20 at timet₄. The base station 20 in reception mode leads the returned code signalto the counter circuit 27 so that the returned number of cycles can becounted in counter 29. At time t₅, the counter 28 counts twice thenumber of cycles corresponding to the code signal, namely 4096. At thistime, the count status of counter 29 is stopped and read bymicrocontroller 30. This count status is at first, for example, 2042.The returned code signal is not yet completely received. This“premature” stopping of counter 29 is used to compensate for possiblyunrecognized cycles during communication between ID transmitter 21 andbase station 20.

The difference between the number of cycles (here: 2048) for the codesignal and the number of cycles (here: 2042) determined by counter 29corresponds to the signal propagation time from the base station 20 tothe ID transmitter 21 and back. At time t₆, the ID transmitter 21 isswitched back to receive mode. At this point there is the possibility ofrepeating the distance determination or continuing the request protocol.

In order also to compensate for unrecognized cycles during transmissionof the code signal from the base station 20 to the ID transmitter 21 andduring the subsequent signal processing, instead of transmitting theexact number of cycles, a slightly greater number of cycles can betransmitted. A refinement of the method is especially expedient, whereindelays are inserted between the different modes of the base station 20or the ID transmitter 21, transmit or receive mode. Without anyadditional measures, the transmitted code signal can feature such asafeguard that base station 20 is switched to its receive mode beforethe return of the code signal from ID transmitter 21. The delays can beprovided in a simple way so that the used counter 28 or 40 iscorrespondingly switched, and so that such a delay is measured like thelength of the transmitted code signal, in the described embodiment, 2048cycles long. During later evaluation, such designed delays are takeninto consideration.

Through the doubled propagation time represented in the embodiment, theresolving power with reference to the distance between the base stationand the ID transmitter can be determined with accuracy that is greaterthan distance determination merely through detection of simple signalpropagation time. This accuracy can be further increased by performingthe distance determination not once but many times, and also atdifferent positions within a transmission protocol. Detection of theactual distance is then done, for example, by averaging the individualdetected distances.

From the description of this embodiment, it is clear that distancedetermination between a base station and an ID transmitter is possibledue to the reliable accuracy corresponding to the resolving power, sothat in this way a reliable recognition of unauthorized path extensionis possible. To increase operation security, different code signals canbe used. A preprogramming of the ID transmitter, for example, through apredetermined number of cycles as the code signal is also possible.

Keyless access authorization control device 47 includes a base station Bwith a transceiver unit 48. The keyless access authorization controldevice 47 is used to check access authorization to a vehicle. Thus, basestation B is ardistanced in a vehicle. Access authorization controldevice 47 is also associated with a mobile ID transmitter ID that iscarried by the person authorized to use the vehicle.

The transceiver unit 48 includes a transceiver 49 for transmitting arequest signal at a high-frequency radio path as well as one or moretransmitters 50 for transmitting the low-frequency radio signal. Thetransceiver 49 is connected to a processor 51 that controls thetransmit-receive actions of the transceiver 49. A phase comparator 52 isconnected to the output of transceiver 49. The other input of the phasecomparator is connected to the low-frequency reference signal 53. Theoutput of PLL circuit 52 is applied to an input of processor 51.However, to realize the method according to the invention, it issufficient to use a receiver instead of transceiver 49.

ID transmitter ID essentially consists of a processor 54 and atransceiver 55 that is connected to LF antenna 56.

During the operation of the access authorization control device 47, arequest signal is transmitted in cycles by transceiver unit 48 invehicle 57 on an LF channel, as shown schematically in FIG. 7. Thetransmission power for transmitting this signal is set so that thissignal can be received by ID transmitter ID within a circle ofapproximately 3 m around the vehicle 57. If the ID transmitter ID iswithin the reception area of this LF request signal, the transmitter isactivated by this signal and switched to its active state. If IDtransmitter ID is ready, it transmits a reply data telegram in ASK mode(amplitude shift keying mode) on an HF channel. At a predeterminedposition within this data telegram, the modulation type of the IDtransmitter is changed from ASK modulation to pure amplitude modulation.The amplitude modulation of the carrier wave used to transmit the replysignal is performed with the request signal received on the LF channelby the ID transmitter ID. The time and the period of thisamplitude-modulated reply signal is agreed upon between base station Band ID transmitter ID and can be changed according to a definedalgorithm for increasing the operation security. If thisamplitude-modulated carrier wave has been transmitted for thepredetermined period by ID transmitter ID, its operating mode changesagain to ASK modulation for data transmission. From this principlerepresentation it is clear that a full-duplex operation can be realizedover time with the operation of access authorization control device 47.

The reply signal transmitted from ID transmitter ID is received bytransceiver 49 of base station B. The analog part of the received datatelegram, namely the amplitude-modulated component, is applied to thePLL circuit 57 and its phase is compared with the phase of theoriginally transmitted request signal. If ID transmitter ID is withinthe operating area, both phases are recognized by the phase comparator52 within a predetermined tolerance as constant relative to each otherif there are no other intermediate relay stations. It is clear that inthis method, a comparison of phases on relative paths is performedwithout determining the respective absolute phases.

LF oscillator 53 is operated free-running in the embodiment shown inFIG. 6 so that its natural frequency deviations can be correspondinglyrecovered also in the demodulated reply signal. Through these measures,the operational security of access authorization control device 47 isfurther increased against manipulation.

Another access authorization control device 58 is reproducedschematically in FIG. 8 in a block circuit diagram. Base station B ofthis access authorization control device 58 essentially consists of atransceiver 59, a modulation unit 60 for producing frequency mixing formodulating the carrier wave of a request signal, and a phase comparator61 for corresponding filtering of the carrier wave modulated with themodulation scheme on a reply channel. The transceiver unit 59 includesHF transmitter 62 for transmitting the carrier wave modulated with amodulation scheme, transceiver 64, wherein the transmitter 62 and thetransceiver 64 are connected to a common transmit-receive antenna 66 bymeans of a combiner network 65. A reference signal 63 is also associatedwith the transceiver 59. A demodulator 67 is also connected to thecombiner network 65 for demodulating a received reply signal. At itsoutput, the demodulator 67 is connected to the phase comparator 61.There is a processor 68 for controlling the transmit-receive processesof transceiver 64 and transmitter 62.

A mobile ID transmitter ID belongs to access authorization controldevice 58 and is carried by the person authorized to use the vehicle.The ID transmitter essentially consists of a processor 69, a transceiver70, a demodulator 71, a combiner network 72, and a transmit-receiveantenna 73. Request signals transmitted by the transceiver unit 59 ofbase station B can be received with the transmit-receive antenna 73 anddemodulated by demodulator 26. At its output, demodulator 71 isconnected to an input of transceiver 70 so that the demodulated requestsignal can be used to modulate the reply signal.

A frequency scheme is supplied by modulation unit 60. A carrier wave fortransmitting a request signal is modulated with this frequency scheme.The modulation frequency scheme consists of a base frequency and severalother frequency components that are each components of the basefrequency divided by two. Such a frequency scheme representing therequest signal is reproduced in a diagram in FIG. 9a. The variation ofthe amplitude curve of this frequency scheme is shown in FIG. 9b.

With this modulation frequency scheme, a carrier wave on an HF channelof, for example, 433 MHz, is modulated and transmitted by means oftransmit-receive antenna 66. If ID transmitter ID is within apredetermined distance to the vehicle, this signal will be received anddemodulated by means of demodulator 71. The returned reply signal of IDtransmitter ID is performed by modulation of a carrier wave of, forexample, 868 MHz, in transceiver 70 with the demodulated request signaland thus, with the original frequency scheme used to modulate therequest signal. This reply signal is transmitted by the ID transmitterand received by the transceiver 59, demodulated and then filtered inphase comparator 61. A diode detector is provided as demodulator 67. Thecorner frequencies of the filter contained in phase comparator 61correspond to the base frequency and to the other frequency components.Through a phase comparison of equal frequency components, the distanceof ID transmitter ID from base station B can be calculated. Depending onthe determined distance, there can be a direct decision on the returnradio path of the request and reply signal, allowing the detection ofwhether the reply signal has been transmitted directly from IDtransmitter ID or whether there is the receipt of a reply signal from anintermediate circuit of an unauthorized path extension.

The distance measurement in this embodiment can also be used to make iteasier to find a vehicle parked, for example, in a parking garage. Inthis case, the transmission of the request signal by the base station Bmust be triggered by ID transmitter ID. Correspondingly, the user can beinformed whether he is getting nearer or farther from his vehicle.

In the following, another embodiment of the invention is described withreference to FIGS. 10-12. A transceiver unit 74 of the base station isassociated with the vehicle that is not shown in greater detail. Thistransceiver unit is used to carry out keyless access authorizationcontrol. Transceiver unit 74 essentially consists of a processor 75, atransceiver 76 for data communication with a mobile ID transmitter, acombiner network 77, and a transmit-receive antenna 78 connected to thisnetwork 77. Data communication with the ID transmitter is performed, forexample, at 868 MHz. This data communication includes request-replydialogs that are used to deduce the authorization of the person carryingthis ID transmitter to open the vehicle depending on a received replycode of the ID transmitter. To perform the method according to theinvention, all that is necessary is for the base station to have areceiver, instead of the transceiver 76 shown in the figures, forreceiving the reply signal transmitted from the ID transmitter.

Transceiver unit 74 also includes a transmitter 79 that operates at 433MHz in the described embodiment. The output of transmitter 79 is appliedto combiner network 77, so that transceiver antenna 78 is used both bytransceiver 76 and also by transmitter 79. Transmitter 79 is used totransmit a linearly frequency-modulated carrier wave as a request signalthat can be used to calculate the distance of the ID transmitterreturning this request signal after the request signal is received as areply signal.

An FM demodulator 80 is also connected to the combiner network 77. Thefrequency-modulated reply signal received by the transmit-receiveantenna 78 is demodulated by this demodulator. The output of demodulator80 is connected to processor 75, so that a comparison of the received,demodulated reply signal with the first transmitted modulated requestsignal is possible through processor 75.

An ID transmitter not shown in greater detail includes, as shown in FIG.11, a processor 81 and a transceiver 82 for data communication. Theoutput of transceiver 82 is connected to the input of a combiner network83. A transmit-receive antenna 85 is also connected to this network 83.In addition, the output of an FM demodulator 84 is connected to network83. This demodulator is used to demodulate a request signal transmittedby transceiver unit 74. The output of FM demodulator 84 is connected toan input of transceiver 82. In this way, the request signal demodulatedby FM demodulator 84 is used directly for modulating transceiver 82. Thereply signal returned by the ID transmitter is then part of theotherwise occurring data communication.

To determine the distance of the ID transmitter from the vehicle or fromtransceiver unit 74, the transceiver unit 74 transmits a request signalin the form of a linearly frequency-modulated carrier wave (433 MHz)through the transmitter 79. The use of a linearly frequency-modulatedcarrier wave is advantageous for later evaluation. When the IDtransmitter is at a defined distance to the transceiver unit 74, thisrequest signal is received, demodulated, and used to modulatetransceiver 82 of the ID transmitter. The reply signal returned on the868 MHz channel from the ID transmitter is received by transceiver 74and demodulated in demodulator 80. Due to the return radio path there isa time delay between the linearly frequency-modulated carrier wave ofthe request signal and the corresponding reply signal. This time delayresults from the return radio path—the longer the return radio path, thelonger the delay. Thus, an undesired path extension can be recognizedfrom a relatively large time delay between the modulation frequencyfunction of the request signal and the corresponding reply signal.

A diagram of a linearly frequency-modulated carrier wave is reproducedin FIG. 12 (the modulation curve is indicated by the continuous line),where time is on the x-axis and frequency on the y-axis. The maximum(f₀+f_(HUBmax)) and the minimum (f₀−f_(HUBmax)) of this frequency curverepresents the frequency swing used to modulate the carrier wave (f₀).The reply signal reflected from the ID transmitter and received anddemodulated by transceiver 74 is indicated by the dashed line, with thecorresponding time delay reflecting the return radio path. A comparisonof the function value of the carrier frequency curve of the requestsignal with the corresponding value of the reply signal at time t₀ canbe done by subtracting the absolute values of the two function values oralso by subtracting the two squared function values. Such a comparisonstep can be realized with minimal hardware expense or also in softwarethrough processor 75. If the difference frequency (Δf) calculated inthis way exceeds a certain value, it means that the return radio path(transceiver unit 74-ID transmitter-transceiver unit 74) is longer thana predetermined functional area that has been set, for example, to be5-10 m around the transceiver unit 74. Thus, the ID transmitter is notin the vicinity of the vehicle; obviously, the radio path has beenmanipulated and lengthened. In this case, access authorization isdenied. If the difference frequency (Δf) is within the toleratedinterval, the ID transmitter is in the functional area, and hence thedesired action is performed: the vehicle doors are unlocked.

The resolution of this method relative to the return radio path isdefined by the selection of modulation frequency or by the frequencyswing. By varying the modulation frequency, the repetition of themeasurement values at regular distances (every 360°) can be prevented.The lowest frequency can be considered as the value for the maximumdistance to be measured; the highest frequency in the frequency schemedefines the resolution of the system.

Summary of Reference Numerals

1 Keyless access authorization control device

2 Base station

3 ID transmitter

4 Transceiver unit

5 HF transmitter

6 HF receiver

7 Microcontroller

8 Cycle counter

9 Start input

10 Stop input

11 AND-gate

12 Transceiver unit

13 HF transmitter

14 HF receiver

15 Microcontroller

16 Data line

17 Key line

18 Evaluation logic

19 Keyless access authorization control device

20 Base station

21 ID transmitter

22 Transmit-receive antenna

23 Signal generator

24 Signal splitter

25 Counter circuit

26 HF circuit

27 Counter circuit

28 Counter

29 Counter

30 Microcontroller

31 Frequency dividing circuit

32 Frequency divider

33 Bandpass filter

34 Amplifier

35 Frequency dividing circuit

36 Transmit-receive antenna

37 HF circuit

38 Amplifier

39 HF circuit

40 Counter

41 Frequency dividing circuit

42 Signal splitter

43 Signal generator

44 Keyless access authorization control device

45 Local oscillator

46 Mixer

47 Keyless access authorization control device

48 Transceiver unit

49 Transceiver

50 NF transmitter

51 Processor

52 PLL circuit

53 Frequency generator

54 Processor

55 Transceiver unit

56 NF antenna

57 Vehicle

58 Keyless access authorization control device

59 Transceiver unit

60 Modulation unit

61 Phase comparator

62 HF transmitter

63 Reference signal

64 Transceiver

65 Combiner network

66 Transmit-receive antenna

67 Demodulator, diode detector

68 Processor

69 Processor

70 Transceiver

71 Demodulator

72 Combiner network

73 Transmit-receive antenna

B Base station

ID ID transmitter

74 Transmit-receive unit

75 Processor

76 Transceiver

77 Combiner network

78 Transmit-receive antenna

79 Transmitter

80 FM demodulator

81 Processor

82 Transceiver

83 Combiner network

84 FM demodulator

85 Transmit-receive antenna

What is claimed is:
 1. A method for checking keyless accessauthorization of an operator, the method comprising: (I) transmitting acode signal from a base station to a mobile identification (ID)transmitter carried by the operator; (II) receiving the code signal fromthe base station with the ID transmitter; (III) performing an actionwith the ID transmitter in response to the ID transmitter receiving thecode signal, wherein the action is indicative of a reply signal that isdetectable by the base station; (IV) detecting the reply signal with thebase station; (V) determining the difference of a signal characteristicbetween a reference code signal monitored at the base station and thereply signal received by the base station, wherein the signalcharacteristic correlates with signal propagation time and changes as afunction of the distance between the base station and the IDtransmitter; and (VI) determining distance between the base station andthe ID transmitter on the basis of a relative determination based on thedifference of the signal characteristic between the reference codesignal monitored at the base station and the reply signal received bythe base station.
 2. The method of claim 1 wherein: step (I) includestransmitting a code signal from the base station to the ID transmitteruntil the reply signal is detected by the base station; step (V)includes counting cycles of the code signal while the code signal isbeing transmitted from the base station to the ID transmitter; and step(VI) includes determining the distance between the base station and theID transmitter based on the counted cycles of the code signaltransmitted from the base station to the ID transmitter.
 3. The methodof claim 2 wherein: step (I) includes transmitting a second code signalfrom the base station to the ID transmitter after the reply signal hasbeen detected by the base station; step (III) includes transmitting areply signal from the ID transmitter to the base station until thesecond code signal from the base station is received by the IDtransmitter; step (V) includes counting cycles of the reply signal whilethe reply signal is being transmitted by the ID transmitter to the basestation; and step (VI) includes determining the distance between the IDtransmitter and the base station based on the counted cycles of thereply signal transmitted by the ID transmitter to the base station. 4.The method of claim 2 wherein: the reply signal detected by the basestation is the code signal transmitted by the base station to the IDtransmitter; step (V) includes counting cycles of the code signaltransmitted from the base station to the ID transmitter until the codesignal has been detected by the base station; and step (VI) includesdetermining the distance between the ID transmitter and the base stationbased on the counted cycles of the code signal transmitted by the basestation to the ID transmitter.
 5. The method of claim 4 wherein: step(I) includes transmitting a carrier wave code signal having apredetermined number of cycles from the base station to the IDtransmitter.
 6. The method of claim 2 wherein: the reply signal detectedby the base station is the code signal transmitted by the base stationto the ID transmitter; step (V) includes counting a first set of cyclesof the code signal as the code signal is being transmitted by the basestation to the ID transmitter and then counting a second set of cyclesof the code signal as the code signal is being detected by the basestation until the first set of counted cycles is equal to twice thesecond set of counted cycles.
 7. The method of claim 2 wherein: step (V)includes counting only every 2^(x)th cycle of the code signal while thecode signal is being transmitted from the base station to the IDtransmitter.
 8. The method of claim 1 wherein: step (I) includestransmitting a code signal with modulation instructions from the basestation to the ID transmitter; step (III) includes transmitting amodulated carrier wave reply signal modulated in accordance with themodulation instructions of the code signal transmitted from the IDtransmitter to the base station; step (V) includes comparing the phaseof the demodulated carrier wave reply signal with the phase of the codesignal.
 9. The method of claim 8 wherein: the carrier wave of the replysignal is amplitude-modulated.
 10. The method of claim 8 wherein: step(I) includes transmitting a code signal at a frequency used to modulatethe reply signal.
 11. The method of claim 10 wherein: step (I) includestransmitting a code signal at a low frequency from the base station tothe ID transmitter; and step (III) includes transmitting a modulatedcarrier wave reply signal amplitude-modulated with the code signaltransmitted from the ID transmitter to the base station.
 12. The methodof claim 10 wherein: step (I) includes transmitting a code signal atdifferent frequencies during transmission from the base station to theID transmitter.
 13. The method of claim 8 wherein: step (I) includestransmitting a code signal with coding from the base station to the IDtransmitter.
 14. The method of claim 8 wherein: step (I) includestransmitting a code signal modulated with a modulation scheme formedfrom at least one modulation variable from the base station to the IDtransmitter; and step (III) includes transmitting a reply signalmodulated in accordance with the modulation scheme from the IDtransmitter to the base station.
 15. The method of claim 14 wherein: themodulation scheme is a frequency scheme having individual frequencycomponents; and step (V) includes comparing the phase of code signalwith the reply signal is performed with reference to individualfrequency components forming the modulation scheme.
 16. The method ofclaim 1 wherein: step (I) includes transmitting a modulated carrier wavecode signal from the base station to the ID transmitter; step (III)includes transmitting a reply signal modulated in accordance with themodulated carrier wave code signal from the ID transmitter to the basestation; step (V) includes comparing modulation function value ofmodulation variables of the reply signal with modulation function valueof modulation variables of the code signal; and step (VI) includesdetermining distance between the base station and the ID transmitterbased on the comparison of the modulation function values.
 17. Themethod of claim 16 wherein: step (V) includes performing a differenceformation of absolute values of simultaneous modulation function values.18. The method of claim 16 wherein: step (V) includes performing adetermination depending on the calculated comparison value (Δf) in asubsequent step, wherein the determination is whether the ID transmitteris within a predetermined receive area relative to the base station. 19.The method of claim 16 wherein: the frequency of the carrier wave of thecode signal transmitted from the base station to the ID transmitter isdifferent than the frequency of the carrier wave of the reply signaltransmitted from the ID transmitter to the base station.
 20. The methodof claim 16 wherein: the modulated carrier wave code signal transmittedfrom the base station to the ID transmitter is linearly-frequencymodulated.
 21. The method of claim 20 wherein: step (III) includestransmitting a modulated carrier wave reply signal modulated inaccordance with the modulated carrier wave code signal from the IDtransmitter to the base station.
 22. The method of claim 16 wherein: themodulated carrier wave signal transmitted from the base station to theID transmitter is amplitude-modulated.
 23. The method of claim 1wherein: the base station is associated with a vehicle.
 24. A device forchecking keyless access authorization of an operator, the devicecomprising: a base station having a processor and a transceiver fortransmitting a code signal and receiving a reply signal; a mobileidentification (ID) transmitter carried by the operator for transmittinga reply signal to the base station in response to receiving a codesignal from the base station; wherein the processor of the base stationdetermines the difference of a signal characteristic between a referencecode signal monitored by the processor and the reply signal received bythe base station, the signal characteristic correlates with signalpropagation time and changes as a function of the distance between thebase station and the ID transmitter; wherein the processor determinesdistance between the base station and the ID transmitter on the basis ofa relative determination based on the difference of the signalcharacteristic between the reference code signal monitored at the basestation and the reply signal received by the base station.
 25. A methodfor checking access authorization of an operator, the method comprising:(I) transmitting a code signal from a base station to a mobileidentification (ID) transmitter carried by the operator; (II) receivingthe code signal from the base station with the ID transmitter; (III)performing an action with the ID transmitter in response to the IDtransmitter receiving the code signal, wherein the action is indicativeof a reply signal that is detectable by the base station; (IV) detectingthe reply signal with the base station; (V) determining the differenceof a distance-dependent signal characteristic correlated with signalpropagation time between a reference code signal monitored at the basestation and the reply signal; and (VI) determining distance between thebase station and the operator based on the difference of thedistance-dependent signal characteristic correlated with signalpropagation time between the reference code signal monitored at the basestation and the reply signal; wherein, step (I) includes transmitting acode signal from the base station to the ID transmitter until the replysignal is detected by the base station; step (V) includes countingcycles of the code signal while the code signal is being transmittedfrom the base station to the ID transmitter; and step (VI) includesdetermining the distance between the base station and the operator basedon the counted cycles of the code signal transmitted from the basestation to the ID transmitter; wherein, step (I) further includestransmitting a second code signal from the base station to the IDtransmitter after the reply signal has been detected by the basestation; step (III) includes transmitting the reply signal from the IDtransmitter to the base station until the second code signal from thebase station is received by the ID transmitter; step (V) includescounting cycles of the reply signal while the reply signal is beingtransmitted by the ID transmitter to the base station; and step (VI)includes determining the distance between the operator and the basestation based on the counted cycles of the reply signal transmitted bythe ID transmitter to the base station.
 26. A method for checking accessauthorization of an operator, the method comprising: (I) transmitting acode signal from a base station to a mobile identification (ID)transmitter carried by the operator; (II) receiving the code signal fromthe base station with the ID transmitter; (III) performing an actionwith the ID transmitter in response to the ID transmitter receiving thecode signal, wherein the action is indicative of a reply signal that isdetectable by the base station; (IV) detecting the reply signal with thebase station; (V) determining the difference of a distance-dependentsignal characteristic correlated with signal propagation time between areference code signal monitored at the base station and the replysignal; and (VI) determining distance between the base station and theoperator based on the difference of the distance-dependent signalcharacteristic correlated with signal propagation time between thereference code signal monitored at the base station and the replysignal; wherein, step (I) includes transmitting a code signal from thebase station to the ID transmitter until the reply signal is detected bythe base station; step (V) includes counting cycles of the code signalwhile the code signal is being transmitted from the base station to theID transmitter; and step (VI) includes determining the distance betweenthe base station and the operator based on the counted cycles of thecode signal transmitted from the base station to the ID transmitter;wherein the reply signal detected by the base station is the code signaltransmitted by the base station to the ID transmitter, and step (V)includes counting a first set of cycles of the code signal as the codesignal is being transmitted by the base station to the ID transmitterand then counting a second set of cycles of the code signal as the codesignal is being detected by the base station until the first set ofcounted cycles is equal to twice the second set of counted cycles.